Additive Manufacturing of High-Entropy Alloys A Review. Wenyuan Cui*, Xinchang Zhang*, Frank Liou*

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1 Solid Freeform Fabrication 2017: Proceedings of the 28th Annual International Solid Freeform Fabrication Symposium An Additive Manufacturing Conference Reviewed Paper Additive Manufacturing of High-Entropy Alloys A Review Wenyuan Cui*, Xinchang Zhang*, Frank Liou* *Department of Mechanical and Aerospace Engineering, Missouri University of Science and Technology, Rolla, MO, Abstract High-entropy alloys have attracted increasingly interest due to their unique compositions, microstructures and mechanical properties. Additive manufacturing has been recognized as a promising technology to fabricate the high-entropy alloys in the recent years. The purpose of this paper is to review the current research progress in high-entropy alloys by additive manufacturing process. It will first highlight the important theory of the high-entropy alloys. The next aspect is to summarize current additive manufacturing methods applied for the high entropy alloys. At last, the correlation between the microstructures and the mechanical properties of the high-entropy alloys will be examined and discussed. Keywords: Additive Manufacturing; High-Entropy Alloys; Microstructures; Mechanical Properties Introduction High-entropy alloys (HEAs) were first proposed by Yeh et al. and have attracted increasingly attention from the researchers and scientists [1] [10]. The conventional metallic alloys usually are choosing one principal element with small amounts of other elements added to improve the properties. By contrast, HEAs are defined as alloys that have at least five principal elements and the concentration of each element is between 5 at. % to 35 at.%. Yeh summarized the core effects for HEAs: high-entropy effect, sluggish diffusion, severe lattice distortion and cocktail effects [1] [3]. The high-entropy effect implies that the high mixing entropy of the elements leads to the formation of single-phase solid solution with simple crystal structures like face-center cubic (fcc) or body-center cubic (bcc) rather than intermetallic. In HEAs, the properties can be influenced by adjusting the composition change and this is called cocktail effect. It has been a challenge for the researchers to investigate the fundamental of HEAs due to the lack of the thermodynamic database and phase diagrams. Current approaches and rules to predict the phases in HEAs are based on the empirical equations like Gibbs energy, mixing entropy and mixing enthalpy [9][11]. Zhang and Yang proposed the parameters of W and atomic size d as a starting point to predict the formation of solid-solution phases [6], [7]. Valence electron concentration (VEC) is another criterion to decide the bcc or fcc crystal structure in HEAs [12]. These parameters have been applied in some alloys systems. The microstructures and mechanical properties of HEAs have been reported indicating their high strength, wear resistance, ductility and corrosion resistance [5], [6], [9], [13] [15]. Most of the work utilized casting or arc melting to produce HEAs. However, it is not easy to overcome the inherent complexity and high degree of controls by the traditional manufacturing methods [16], 712

2 [17]. Moreover, it is unlikely to design the industrially suitable route to mass produce HEAs. As a result, additive manufacturing (AM) with a higher level of local process and rapid solidification rate will be a promising way to produce HEAs as engineering materials [18], [19]. Wu et al. successfully synthesized FeCoCrAlCuNi x, FeCoCrAlNiTi x, FeCoCrAlNi HEAs coatings free of cracks and pores by laser surface alloying [20] [22]. Selective laser sintering (SLM) was chosen to produce FeCoCrNi HEA and it showed high strength and ductility comparable to the stainless steel [16]. AlCoCrFeNi HEA produced by selective electron beam melting (SEBM) demonstrated six times higher fracture strength than that of a conventional engineering material [17], [23]. This work aims to provide a thorough literature review on the powder feed and powder bed laser additive manufacturing process of the common HEA parts. It will first introduce the general concept of HEAs design and unique characteristics of powder feed and powder bed laser additive manufacturing. Furthermore, the bulk of the work will focus on the evaluation of microstructures, mechanical properties together with the corrosion resistance of HEAs. Theory of High-Entropy Alloys The prediction and the stability of the solid solutions have been a topic of significant importance in current HEAs research. Thermodynamics parameters like Gibbs energy, entropy and enthalpy among the phase competition in HEAs were delicately discussed [8], [24]. For the random solid-solution system of N component, the configuration entropy can be expressed in Eq. (1). Since HEAs possess more than five principal components, it increases the extent of confusion in the system and mixing entropy. A common approach is the utilization of Hume-Rothery rule, which the atomic size (d) and the mixing enthalpy ( H #$% ) are the two domain factors to form substitutional solid solutions. Zhang et al. proposed the parameter d to study the relationship between the phase stability and the difference in atomic size between the N-element compositions Eqs. (2), (3) [5] [7]. + S #$% = R $,- C $ lnc $ (1) + H #$% = 4 $,-,$31 H #$% $1 C $ C 1 (2) + $,- + $,- c $ r ) < (3) $ δ = 100 C 7 (1 r $ where H #$% are the mixing enthalpy between the A element and B element, c $ is the molar concentration of the ith component and r $ is the atomic radius of the ith component. It indicates that the solid-solution formation requires δ 6.6%. Yang et al. established another parameter W which defines the combined effects between S #$% and H #$% used to predict the solid-solution phase formation in HEA [7], [9]. Ω = C D E DFG I DFG (4) + T # = $,- c $ (T # ) $ (5) 713

3 where T # is the average melting temperatures of the N-components in the alloy system. By analyzing the equations above, Yang pointed out that W=1 is a critical value to form the solid solution. For W>1, the contribution of H #$% is less than that of T # S #$% and the multicomponent system consists of the solid solutions. Guo et al. studied the stability of face-center cubic(fcc) and body-center cubic(bcc) solid solutions are determined by valence electron concentration (VEC) [12]. A higher valence electron concentration (VEC³8) favors the fcc solid solutions and bcc solid solution is stabilized at a lower value (VEC<6.87). The VEC values of HEAs are expressed in Eq. (6). + VEC = $,- C $ (VEC) $ (6) where c $ is the molar concentration for the ith component, (VEC) $ is the valence electron concentration for the individual ith composition. Table 1 illustrates the alloy composition, the calculated values of d, VEC, W and crystal structures reported by additive manufacturing. It can be seen that the formation of fcc or bcc solid solution is in good agreement with the calculated parameters for most of the cases. However, the crystal structure in a few alloys does not coincide with the parameter d. For example, FeCoCrAlNiTi 1.5 and FeCoCrAlNiTi 2.0, the metastable phase Ti 2 Ni was reported along with fcc and bcc structures. This phenomenon could be mainly attributed to the improved solubility and a uniform compositional distribution in the rapid solidified laser coating. Moreover, it is pointed out that the rapid solidification reduces the solid-liquid surface energy of the metastable phases and stimulate the formation of the metastable phases in the alloy systems. Table 1 HEA crystal structures and thermal parameters reported by AM Alloy ẟ VEC Ω Crystal Structure Ref. FeCoCrAlNiTi fcc, bcc [20] FeCoCrAlNiTi fcc, bcc [20] FeCoCrAlNiTi fcc, bcc, Ti 2 Ni [20] FeCoCrAlNiTi fcc, bcc, Ti 2 Ni [20] AlFeCoCrNi bcc [25] CoCrAlNiFe bcc [22] NiCrCoTiVAl bcc, α-ti [19] AlCrFeNiCoCu bcc, fcc [8] FeCoCrAlCuNi fcc, bcc [21] FeCoCrAlCuNi fcc, bcc [21] FeCoCrAlCuNi fcc [21] FeCoCrAlCuMo fcc, bcc, Fe 2 Mo [26] FeCoCrAlCuSi fcc, bcc [26] FeCoCrAlCuTi fcc, bcc, Fe 2 Ti [26] 714

4 Additive Manufacturing Methods Most metallic powder additive manufacturing methods can be classified into three categories, (1) powder feed, (2) powder bed fusion and (3) wire feed process [23]. Most commonly used methods in HEAs are powder feed and powder bed fusion methods. A set up of direct metal deposition (DMD) with coaxial powder feeder is shown in Figure 1. During the process, a concentrated laser beam melts the synchronously fed powders and a thin layer of the substrate. The metallic powders undergo a rapid melting and cooling process. Meanwhile, a metallurgical bonding can be formed between the substrate and the powders. Al x FeCoCrNi x (x=0.3,1) HEAs were fabricated by DMD [27]. Similar method of laser cladding is also applied in FeCoCrAlCu [21], Al x FeCoNiCuCr [28]. Kunce used the direct laser-engineered net shaping (LENS) for HEA as a hydrogen storage material [29]. Figure 1 Schematic diagram of direct metal deposition (DMD) process setup[30] For the laser-based additive manufacturing methods, the precursor metallic powders are melted by high energy laser or electron beam in a layer by layer manner[16], [31], [32]. A schematic selective laser melting (SLM) layout is present in Figure 2. The geometrically complex components with high dimension precision can be obtained by the SLM process without subsequent process. The inert Argon or Nitrogen is usually used as the atmosphere environment. Fujieda first reported the feasibility of this method used for AlCoCrFeNi HEA, which facilitated a high level of local process control and generated rapid solidification rates [17]. It showed the ductility and fracture strength were remarkably improved compared to the conventional methods. It was further explained the preheating process led to low residual stress and significantly affected the microstructures and mechanical properties[23], [33], [34]. 715

5 Figure 2 A typical selective laser melting (SLM) machine layout [31] Microstructure Characterization Microstructures and mechanical properties of HEAs have been frequently investigated. As the metallic powders characteristics have critical an impact on the mechanical properties, some amount of studies have been conducted in this area. The pure elemental powders of Fe, Co, Ni, Cr and Cu (99.7% wt.% minimum) were mechanically mixed together and the particle sizes were about µm [31] Additives of Si and Mo were added into the FeCoNiCrCu powders as master alloys in order to avoid the different melting points and densities of nonmetallic elements. The morphology of the AlCoCrFeNi powders used for the HEA is in Figure 3. It was observed that the powder particles were spherical or nearly spherical with relatively smooth surface. But a few aggregates of the powder particles were formed due to the turbulent flow in the atomization chamber and the collision of the solidifying droplets [29]. Figure 3 Morphology (a), SEM/BSE micrographs (b) of the AlCoCrFeNi powder particles[29] Figure 4 shows the SEM/BSE images of the AlCoCrFeNi HEAs cast and SEBM specimens. The images of the SEBM specimens were obtained at the cross-section perpendicular to the build direction. It is seen the cast specimens have much coarser grain sizes than the SEBM 716

6 counterparts (Figure 4 a-c). The SEBM specimens present the equiaxed grains, but the more coarsening of the modulated structure was seen at the bottom of the part. Figure 4 SEM images of the (a) cast, the SEBM (b) top and (c) bottom of AlCoCrFeNi HEA [23] Mechanical Properties Mechanical properties of HEAs have been studied from the aspects of hardness, tensile and yield strength testing. The studies indicate that the mechanical properties of additive manufactured HEAs are comparable or better than the conventional alloys [24], [35] [37]. Moreover, the process parameters, alloying element and post-treatment may cause remarkable effect on the mechanical properties. Vickers hardness testing has been widely used in mechanical property characterization of HEAs. Vickers hardness of the most common HEAs are compared with the conventional Ni, Al and Fe and Ti based alloys in Figure 5. The hardness varies in one alloy system. For instance, the Al x FeCoNiCuCr system, the hardness varies from 390HV to 687HV depending on the contents of Al. The hardness of several types of HEAs like AlCoCrFeCu, AlCrFeCoCuTiNi and AlCoCrFeNiTi are generally higher than the conventional alloys. 717

7 Figure 5 Hardness of most common HEAs compared with the conventional alloys using the data from [9], [20], [22], [29], [38] [44] The selection of alloys system is critical in determining the hardness of HEAs. The hardness has the trend of increase with the Ni content in Figure 6. Since more bcc crystal structure with higher microhardness formed within the alloy system, it enhances the corresponding microhardness of the HEA [40]. The addition of Al promotes the transition from fcc to bcc crystal structure in Al x FeCoNiCuCr system reported [28]. As the bcc is considered to be higher in microhardness than fcc structure, it shows a sharp increase when the X increases from 1.0 to 1.5. However, the addition of Al also increases the cracks. It indicates that the optimal X lies between 1.5 and 1.8 for a better balance of the microhardness and cracks. Process parameters have significant effect on the hardness. It was found the average hardness of laser-produced AlCoCrCrNi samples were approximately 13% higher than the as-cast state [29]. This is due to the grain refinement of the LENS-produced samples. It also found that the microhardness increases with the cooling rate during the laser process. Zhang discussed that the rapid solidification during the laser process could suppress the grain growth and hence increase the amount of grain boundaries and form the grain boundary strengthening [39]. 718

8 Ni0 Ni0.5 Ni1.0 Ni1.5 Ni2.0 Microhardness (HV) Distance to surface (mm) Figure 6 Microhardness of Al 2 CrFeCoCuTiNix high-entropy alloys using the data from [40] Compressive tests are also applied to evaluate the mechanical properties of laser-produced HEAs in Table 2. It reported the overall compressive properties of AlCoCrFeNi specimens by SEBM were superior to those casting specimens [17]. The enhanced plasticity of SEBM specimens could be ascribed to the finer microstructure and existence of fcc phases, which had higher atomic packing factor and closer packed slip planes. Post heat treatment was conducted on the DML fabricated AlCoCrFeNi samples by Wang [45]. The softer fcc phase resulted in reduced yield strength with enhanced ductility and this was proved in their fractographic analysis. Table 2 Compressive properties of HEAs at room temperature Alloy σ y (Gpa) σ max (GPa) ε p (%) Process Ref. AlCoCrFeNi DML, 600 C/168h HT [45] DML, 800 C/168h HT [45] DML, 1000 C/168h HT [45] DML, 1200 C/168h HT [45] AlCoCrFeNi 1.308± ± ±1.9 Casting [17] 1.015± ± ±6.7 1 SEBM (0 ) [17] 0.944± ± ±5.3 2 SEBM (90 ) [17] 1 parallel to laser build direction, 2 perpendicular to laser build direction. 719

9 Corrosion Resistance The corrosion features of HEAs is determined by the interaction between the environment and materials [20], [46] [48]. The common service environments include the salt water or acid. The corrosion-resistance of HEAs are usually affected by the alloying element, phase change, phase segregation. Some recent working concerning on corrosion-resistance of HEAs will be discussed here. Wu et al. investigated the effect of Ni content in the alloy of FeCoCrAlCuNi x and Al 2 CrFeCoCuTiNi x coatings by laser cladding [20], [40]. SEM results showed the coating structure was consisted of heat-affected, bounding and cladding zones. Simple microstructure of fcc and bcc phases with columnar crystals were observed, which decreased the ingredient segregation. The corrosion current reduced 1 magnitude compared to Q235 carbon steel. Al 2 CrFeCoCuTiNi 1.0 exhibited the best resistance to corrosion in the tests. However, with the increase of Ni content, the corrosion resistance enhanced first and then dropped. Wu explained that the Ni has relatively small atomic size and its lattice was seriously distorted at high contents [20]. Ye et al. studied the corrosion properties of Al x FeCoNiCuCr coatings on 314L stainless steel [28], [38]. The coating had good metallurgical bonding with 314L stainless steel substrate. The relative uniform element distribution within the coatings showed better corrosion resistance than 314 L stainless steel in 0.05M HCl solution, which could be attributed to the additional of Al element. The process parameters like laser scan speed also influenced the corrosion resistance of HEAs. This can be explained that the higher the scan speed, the less laser energy was absorbed by the HEAs [13]. Therefore, the fast cooling rate made the layer surface smooth and uniform and this helped the corrosion resistance of HEAs. Table 3 summaries the electrochemical parameters through the potentiodynamic polarization tests for HEAs in aqueous environments at room temperature. As discussed above, the contents of Al and Ti elements benefit the corrosion resistance of HEAs. The addition of Ni increases the corrosion resistance first and then decreases. Table 3 Electrochemical parameters in aqueous environment of HEAs at room temperature Alloy I 1 corr/acm -2 E 2 corr/v SCE Solution Ref. FeCoCrAlCuNi wt.%nacl [21] FeCoCrAlCuNi wt.%nacl [21] FeCoCrAlCuNi wt.%nacl [21] Al 2 CrFeCoCuTiNi wt.%nacl [40] Al 2 CrFeCoCuTiNi wt.%nacl [40] Al 2 CrFeCoCuTiNi wt.%nacl [40] Al 2 CrFeCoCuTiNi wt.%nacl [40] Al 2 CrFeCoCuTiNi wt.%nacl [40] Al 1.0 FeCoNiCuCr M HCl [28] 720

10 Al 1.3 FeCoNiCuCr M HCl [28] Al 1.5 FeCoNiCuCr M HCl [28] Al 1.8 FeCoNiCuCr M HCl [28] Al 2.0 FeCoNiCuCr M HCl [28] FeCoCrAlNiTi wt.%nacl [20] FeCoCrAlNiTi wt.%nacl [20] FeCoCrAlNiTi wt.%nacl [20] FeCoCrAlNiTi wt.%nacl [20] FeCoCrAlCuMo wt.%nacl [26] FeCoCrAlCuSi wt.%nacl [26] FeCoCrAlCuTi wt.%nacl [26] 1 I corr : corrosion current density; 2 E corr : corrosion potential, V SCE : potential versus saturated calomel electrode. Conclusions HEAs are emerging class of metallic materials. The thermodynamic theory and current additive manufacturing methods to fabricate HEAs have been reviewed in this paper. Due to the advantages of additive manufacturing, HEAs possess superior mechanical and corrosionresistance properties to the conventional alloys. The selection of alloy element and controlling the process parameters are relatively new research topics that hold promise of future HEAs research. Acknowledgement This project was supported by National Science Foundation Grants CMMI and CMMI , and the Intelligent Systems Center, Center for Aerospace Manufacturing Technologies, and Material Research Center at Missouri S&T. Their financial support is greatly appreciated. Reference [1] B. J. Yeh et al., Nanostructured High-Entropy Alloys with Multiple Principal Elements : Novel Alloy Design Concepts and Outcomes **, no. 5, pp , [2] J. Yeh, Y. Chen, S. Lin, and S. Chen, High-Entropy Alloys A New Era of Exploitation, vol. 560, pp. 1 9, [3] J. W. Yeh, Alloy design strategies and future trends in high-entropy alloys, Jom, vol. 65, no. 12, pp , [4] M. Chuang, M. Tsai, W. Wang, S. Lin, and J. Yeh, high-entropy alloys, vol. 59, pp , [5] Z. Tang et al., Aluminum alloying effects on lattice types, microstructures, and mechanical behavior of high-entropy alloys systems, Jom, vol. 65, no. 12, pp , [6] Y. Zhang, X. Yang, and P. K. Liaw, Alloy design and properties optimization of highentropy alloys, Jom, vol. 64, no. 7, pp , [7] X. Yang and Y. Zhang, Prediction of high-entropy stabilized solid-solution in multi- 721

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